This invention relates to microporous membranes prepared from poly(etheretherketone)-type polymers and a process for making the same. Such membranes are useful in the treatment of liquids by the membrane separation processes of ultrafiltration, microfiltration, membrane distillation, and membrane stripping. The membranes of this invention are also useful as microporous supports for composite liquid or gas separation membranes.
In the past, microporous membranes have been fabricated from polyolefins such as polyethylene and polypropylene. One typical method of preparing these membranes is by an extrusion process which involves dissolving the polyolefin in a solvent or a mixture of solvent and non-solvent, extruding the polymer/solvent/non-solvent mixture into membranes, and immersing the membranes into a leach bath. Another method of preparing these membranes is by a melt-extrusion process which involves extruding the membranes from the molten polyolefin, followed by cold drawing the membranes. However, polyolefins, while inexpensive and easy to process, exhibit relatively low heat distortion temperatures.
Poly(etheretherketone)-type polymers are high performance thermoplastics which possess high glass transition temperatures, high crystalline melting points, high thermal stability, and high solvent resistance. These properties make poly(etheretherketone)-type polymers useful for membranes used in liquid separations, particularly membrane separation processes which involve treatment of organic, acidic, or basic liquids at elevated temperatures.
The very properties which make poly(etheretherketone)-type polymers desirable materials for use in applications which require high temperature and/or solvent resistance also make the polymers very difficult to process into membranes. Poly(etheretherketone)-type polymers are extremely solvent resistant and are therefore considered to be insoluble in all common solvents. Therefore, to form membranes., for example, poly(etheretherketone) is typically dissolved in very strong organic acids such as concentrated sulfuric acid to sulfonate the poly(etheretherketone), which renders the sulfonated poly(etheretherketone) soluble in common solvents such as dimethylformamide and dimethylacetamide. The problem with this process is that the polymer in the fabricated membrane is not poly(etheretherketone), but rather sulfonated poly(etheretherketone), which is soluble in common solvents. Furthermore, sulfonated poly(etheretherketone) swells in aqueous solutions, which adversely affects membrane performance in aqueous separation applications.
What is needed is a process of preparing microporous poly(etheretherketone)-type membranes using plasticizers which do not chemically modify or degrade the poly(etheretherketone)-type polymer during fabrication so that the high strength, temperature resistance, and solvent resistance of the unsulfonated poly(etheretherketone)-type polymer is retained by the fabricated membranes.